This invention relates to a novel structure of electrodes of a vacuum switch of the type in which an axial magnetic field is generated.
As is well known in the art, a vacuum switch generally comprises a pair of electrodes disposed in the vacuum switch to be relatively movable to interrupt current passing. With the vacuum switch it is difficult to provide the electrodes in a mutually slidable manner in the vacuum condition, so that the electrodes are ordinarily held in an abutting relation. However, electrodes held in contact condition in the vacuum tend to be fused together particularly when the vacuum switch is thrown into ON state due to the arc created at that time or by a heavy current flowing between the electrodes. In order to prevent such adverse mutual fusion of the electrodes, contacts made of a metal having a fusion preventing property or a copper alloy containing various additives have been provided on the contacting surfaces of the electrodes. Ordinarily, the metal and additives are selected from those having vapor pressures higher than that of copper at the same temperature condition. As is well known in the art, the arc created in a vacuum switch at the time of interruption is sustained by the vapor of metals forming the electrodes. Thus, in case of a vacuum switch wherein arc is stabilized by the application of an axial i.e. longitudinal magnetic field, if an uneven distribution exists in the material forming the electrodes, the arc created between the electrodes tends to be concentrated into the unevenly distributed portion of the material. Furthermore, the alloy containing additives tends to create metallurgical defects, which in turn deteriorate the dielectric strength and reduce the interruption capability of the vacuum switch.
For this reason, it is preferable to reduce the surface area of the contacts to have the smallest possible value. However, an excessive reduction of the surface area is liable to concentrate the arc into limited portions A of the contacts 1 and 2 as shown in FIG. 1, thus heating and/or melting the portions A and reducing the interruption capability of the vacuum switch.
In order to avoid the above described difficulty, an alternative construction as shown in FIG. 2 may be considered, in which the contacting areas of the contacts 1 and 2 are increased so as to cover the entire surfaces of the electrodes 3 and 4, respectively. In this case, although a wide distribution of arc and an increased interruption capability of the vacuum switch may be expected, the deterioration of the dielectric strength due to the defects of the contact material becomes intolerably large.
Conversely, when a material having a vapor pressure lower than that of copper is used for the contacts, the arc is concentrated to a portion of the electrodes other than the contacts, thus deteriorating the current interrupting capability of the vacuum switch.